In general, linear non-crosslinked polymers are thermoplastic in nature and exhibit continuous flow under the influence of heat and pressure. Such polymers can be resoftened as often as desired and are usually soluble in selected solvents. In contrast, crosslinked polymers are generally thermoset, i.e., incapable of being resoftened without decomposition, and are insoluble in most solvents. A linear polymer may nevertheless contain a small number of crosslinkages without losing its thermoplastic properties. It is often desirable to convert thermoplastic polymers into thermoset, crosslinked or partially crosslinked polymers in order to decrease their solubility and flow properties and to obtain a harder and tougher product. Crosslinking of thermoplastic polymers to prepare elastomers therefrom is commonly referred to as vulcanization.
Among the thermoplastic polymers which are more difficult to vulcanize are those prepared by the polymerization of halogenated monoolefins, such as, for example, the copolymers of chlorotrifluoroethylene and vinylidene fluoride, the copolymers of perfluoropropene and vinylidene fluoride and the copolymers of perfluoropropene, vinylidene fluoride and tetrafluoroethylene. Many of these fluorinated thermoplastic polymers have unique and valuable properties such as acid and base resistance and thermal stability. When it is desired to retain their unique properties and at the same time decrease thermoplastic flow and solubility, the polymers are crosslinked.
The use of various materials to vulcanize or cure halogenated monoolefins has been suggested in a number of issued patents. For example, polymers of halogenated monoolefins, especially fluorinated monoolefins, have been vulcanized with curing systems which contain quaternary ammonium or phosphonium compounds, e.g., U.S. Pat. Nos. 3,655,727, 3,712,877, 3,844,877, 3,933,732. The performance characteristics of the cured fluoroelastomers produced using the above conventional vulcanizing agents are adequate for most applications. However, conventionally vulcanized fluoroelastomers while being stable under all but the most extreme chemical and thermal environments are found to be susceptible to degradation in certain high performance applications involving long term exposure to elevated temperatures, especially in harsh chemical environments, e.g., expansion joints in flue gas systems. The present invention provides novel vulcanizing agents which cure polymers of fluoromonoolefins in much the same manner as conventional vulcanizing agents yet unexpectedly provides cured fluoroelastomers with improved long term thermal stability, especially in the presence of harsh chemical environments.